1. Field of the Invention
The present invention relates to a mobile communication system. More particularly, the present invention relates to a method for transmitting a physical channel in a Time Division Duplex (TDD) communication system capable of carrier aggregation which supports aggregation of carriers having different TDD configurations.
2. Description of the Related Art
Recently, research is being actively conducted on Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier-Frequency Division Multiple Access (SC-FDMA) as high speed data transmission techniques over radio channel. Multiple access techniques are used to allow multiple users to share the radio spectrum by allocating distinct time-frequency resources carrying data or control information to respective users, i.e., maintaining orthogonality.
One of the significant features of the mobile communication system is to support scalable bandwidth for providing a high speed wireless data service. For example, the Long Term Evolution (LTE) system is capable of supporting various bandwidths, e.g., 20/15/5/3/1.4 Mhz. Meanwhile, the LTE-Advanced (LTE-A) system can support high data rate transmission over a wide bandwidth up to 100 MHz for a single User Equipment (UE) with carrier aggregation. The mobile carriers can provide their services with one of the available bandwidths. A UE can operate with various capabilities of minimum 1.4 MHz bandwidth up to 20 MHz bandwidth.
In order to support the high data rate transmission, the LTE-A system uses bandwidth wider than that of the LTE system while preserving backward compatibility to the legacy systems for supporting the LTE UEs. This means that it is required for the LTE terminals to attach to the LTE-A system to receive service.
For the backward compatibility, the system bandwidth of the LTE-A system is divided into a plurality of sub-bands or Component Carriers (CC). In the LTE-A system, the component carriers are aggregated for the high data rate transmission for transmission/reception of the data generated per component carrier. Accordingly, the LTE-A system is capable of providing LTE UEs with high speed data transmission service using the transmission/reception processes of legacy LTE system per component carrier.
The component carriers or cells are classified into a Primary Cell (PCell) and a Secondary Cell (SCell) according to their importance in view of the UE. From the UE's viewpoint, there is one PCell, and other cells of the UE are SCells. In the current LTE-A system, the uplink control channel can be transmitted only in the PCell while the uplink data channels can be transmitted in both the PCell and the SCells.
Typically, the scheduling information for the data to be transmitted on the component carriers is transmitted to the UE in Downlink Control Information (DCI). The DCI is generated in a different DCI format according to whether scheduling information is of uplink or downlink, whether the DCI is compact DCI, whether spatial multiplexing with multiple antennas is applied, and whether the DCI is the power control DCI. For example, the DCI format 1 for the control information about downlink data to which Multiple Input Multiple Output (MIMO) is not applied includes the following control information.                Resource allocation type 0/1 flag: It notifies the UE of whether the resource allocation type is type 0 or type 1. Here, type 0 indicates resource allocation in a unit of Resource Block Group (RBG) in bitmap method. In LTE and LTE-A systems, the basic scheduling unit is a Resource Block (RB) representing time and frequency resource, and RBG includes a plurality of RBs and basic scheduling unit of type 0. Type 1 indicates allocation of specific RB in RBG.        Resource block assignment: It notifies the UE of RB allocated for data transmission. At this time, the resource expressed according to the system bandwidth and resource allocation scheme is determined.        Modulation and coding scheme: It notifies the UE of modulation scheme and coding rate applied for data transmission.        Hybrid Automatic Repeat reQuest (HARQ) process number: It notifies the UE of a HARQ process number.        New data indicator: It notifies the UE of whether the transmission is a HARQ initial transmission or retransmission.        Redundancy version: It notifies the UE of redundancy version of a HARQ.        Transport Power Control (TPC) command for a Physical Uplink Control CHannel (PUCCH): It notifies the UE of a power control command for PUCCH as uplink control channel.        
The DCI is channel-coded and modulated and transmitted through Physical Downlink Control Channel (PDCCH).
FIG. 1 illustrates a self-scheduling scheme according to the related art. FIG. 1 illustrates a case where an evolved Node B (eNB) schedules downlink data for a UE with two aggregated carriers (CC#1, CC#2) in the LTE system.
Referring to FIG. 1, a DCI 1 101 to be transmitted on a CC#1 109 is generated with a format defined in the legacy LTE, and channel coded and interleaved as denoted by reference number 103 so as to be carried in PDCCH 105. The PDCCH 105 carries the scheduling information about the Physical Downlink Shared CHannel (PDSCH) 107 as the data channel allocated to the UE on the CC#1 109.
A DCI 111 transmitted on a CC#2 119 is formatted as defined in the legacy LTE standard, channel-coded, and interleaved as denoted by reference number 113 to generate PDCCH 115. The PDCCH 115 carries the scheduling information about a PDSCH 117 as the data channel allocated to the UE on the CC#2 119.
In the LTE-A system supporting carrier aggregation, the data and/or DCI for supporting the data transmission can be transmitted per component carrier as shown in FIG. 1. In order to secure a high reception reliability of the UE, DCI can be transmitted on another component carrier different from the component carrier carrying the data. This is referred to as cross-carrier scheduling and described with reference to FIG. 2.
FIG. 2 illustrates a cross-carrier scheduling scheme according to the related art. FIG. 2 illustrates a case of the scheduling operation to a LTE-A UE capable of using aggregated component carriers CC#1 209 and CC#2 219.
Referring to FIG. 2, the CC#2 219 experiences significant interference compared to CC#1 209 such that it is difficult to satisfy a predefined DCI reception performance requirement for data transmission on the CC#2 219. In this case, an eNB may transmit the DCI on the CC#1 209 under the assumption that the UE knows that the DCI carrying the scheduling information about the data transmitted on CC#2 219 is transmitted on the CC#1 209.
Since any error occurring in data transmission can be corrected later through HARQ, there is no problem in transmitting data on the CC#2 although significant interference exists thereon. In order to make it possible to operate as above, the eNB needs to transmit a Carrier Indicator (CI) indicating the component carrier targeted by the DCI along with the DCI indicating the resource allocation information and transmission format of the scheduled data. For example, CI=‘000’ indicates CC#1 209 and, CI=‘001’ indicates CC#2 219.
Accordingly, the eNB combines a DCI 201 indicating resource allocation information and transmission format of the scheduled data 207 and carrier indicator 202 to generate an extended DCI, performs channel coding, modulation, and interleaving as denoted by reference number 203 on the extended DCI to generate PDCCH, and maps the PDCCH to the PDCCH region 205 of CC#1 209. The eNB also combines a DCI 211 indicating the resource allocation information and transmission format of data 217 scheduled on CC#2 219 and a carrier indicator 212 to generate an extended DCI, performs channel coding, modulation and interleaving as denoted by reference number 213 on the extended DCI to generate PDCCH, and maps the PDCCH not to a PDCCH region 215 of CC#1 219 but to a PDCCH region 205 of CC#1 209.
The TDD system uses a common frequency for uplink and downlink which are discriminated in time domain. In the LTE TDD system, the uplink and downlink signals are discriminated by subframe. In the LTE system, the subframe has a length of 1 ms, and 10 subframes form a radio frame.
The distribution of uplink and downlink subframes can be adapted to the traffic load so as to be in a time domain symmetrically (i.e., an equal number of DL and UL subframes) or asymmetrically (i.e., downlink heavy or uplink heavy).
TABLE 1Uplink-downlinkSubframe numberconfiguration01234567890DSUUUDSUUU1DSUUDDSUUD2DSUDDDSUDD3DSUUUDDDDD4DSUUDDDDDD5DSUDDDDDDD6DSUUUDSUUD
Table 1 shows TDD configurations (TDD uplink-downlink configurations) defined in an LTE standard. In Table 1, subframe numbers 0 through 9 indicate the indices of subframes constituting one radio frame. Here, ‘D’ denotes a subframe reserved for downlink transmission, denotes a subframe reserved for uplink transmission, and ‘S’ denotes the special subframe.
The Downlink Pilot Time Slot (DwPTS) can carry the downlink control information as the normal subframe does. If the DwPTS is long enough according to the configuration state of the special subframe, it is possible to also carry the downlink data. The Guard Period (GP) is the interval required for downlink-to-uplink switch and its length is determined according to the network configuration. The Uplink Pilot Time Slot (UpPTS) can be used for transmitting UE's Sounding Reference Signal (SRS) for uplink channel state estimation and UE's Random Access CHannel (RACH).
For example, in a case of TDD uplink-downlink configuration#6, the eNB can transmit downlink data and/or control information at subframes #0, #5, and #9 and uplink data and/control information at subframes #2, #3, #4, #7, and #8. Here, # indicates a number or an index. Special subframes #1 and #6 can be used for transmitting downlink control information and/or downlink data selectively and SRS or RACH in uplink.
Since the downlink or uplink transmission is allowed for a specific time duration in the TDD system, the timing relationship among the uplink and downlink physical channels needs to be defined, such as control channel for data scheduling, scheduled data channel, and HARQ ACKnowledgement/Non-ACKnowledgement (ACK/NACK) channel corresponding to the data channel.
As described above, when multiple cells are operating with different TDD UL-DL configurations, the HARQ ACK/NACK transmission which has to be carried in a specific uplink subframe of the PCell does not occur synchronized to the TDD UL-DL configuration.
Therefore, a need exists for a method for transmitting a physical channel in a TDD communication system capable of carrier aggregation which supports aggregation of carriers having different TDD configurations.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.